Coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of a sample. Optical Coherence Tomography (OCT) is one example technology that is used to perform usually high-resolution cross sectional imaging. It is applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from the tissue, in vivo, ex vivo or in vitro, and a computer produces images of cross sections of the tissue by using information on how the waves are changed upon reflection.
The original OCT imaging technique was time-domain OCT (TD-OCT), which used a movable reference mirror in a Michelson interferometer arrangement. In order to increase performance, variants of this technique have been developed using two wavelengths in so-called dual band OCT systems.
In parallel, Fourier domain OCT (FD-OCT) techniques have been developed. One example uses a wavelength swept source and a single detector; it is sometimes referred to as time-encoded FD-OCT (TEFD-OCT) or swept source OCT. Another example uses a broadband source and a spectrally resolving detector system and is sometimes referred to as spectrum-encoded FD-OCT or SEFD-OCT.
In scanning OCT, a light beam is focused onto the sample under test by a probe. Returning light is combined with light from a reference arm to yield an interferogram, providing A-scan or Z axis information. By scanning the sample relative to the probe, linear or two dimensional scans can be used to build up a volumetric image. One specific application involves the scanning of arteries, such as coronary arteries. The probe is inserted to an artery segment of interest using a catheter system. The probe is then rotated and drawn back through the artery to produce a helical scan of the inner vessel wall.
Traditionally, scanning OCT probes have been constructed from gradient refractive index (GRIN) lens and fold mirrors. Optical fibers are used to transmit optical signals to the probe at the distal end of the catheter system. The GRIN lens at the end of the fiber produces a collimated or focused beam of light and focuses incoming light onto the end of the optical fiber. The fold mirror couples to the GRIN lens to a region lateral to the probe.